1. Field of the Invention
The invention relates to a method of converting a digital color image to a gray value image, and more particularly, to a method of this kind suitable for distinguishably reproducing colors present in the color image.
2. Discussion of Related Art
A color image is very frequently converted to a gray value image in practice. This is done, for example, when a color photograph is copied by a conventional copier in black and white, or when a color image is printed in black and white by means of a printer.
In modern copying machines, the image on the photograph is converted by a scanner or other image sensing device to a digital image. In many cases the scanner involved converts the image to a digital color image. An image is converted into pixels with a color value for each pixel. It is conventional to express the color values in this case in R, G, B values. To make a gray value print, the copier will convert the R, G and B values to a gray value. For this purpose, the R, G and B values are often averaged, possibly with weighting. This is described, for example, in European Patent Application Publication No. EP-A-0-500327. Preferably, use is then made of the luminance of the color signal. These color-to-gray value conversions based on luminance, and conversions in which (a weighted) average of the color values such as R, G, B is calculated, are hereinafter referred to as “naïve” conversions. A conversion of this kind disregards the context or the environment of a pixel for conversion.
In the conversion of color values to gray values, three dimensions are projected on one dimension. As a result, certainly in the case of the above-described averaging of RGB values, very different colors can be projected on the same or substantially the same gray value. As a result, objects of different color may no longer be distinguishable in the gray value image. Accordingly there is a need for a conversion method which makes color differences which are present in the color image, distinguishable in the gray value image as well.
This requirement is described in U.S. Pat. No. 4,369,461, and the solution proposed therein is to replace colors in the color image by black and white patterns, for example hatching, which can be satisfactorily distinguished from one another.
The above document EP-A-500327 offers a similar solution, in which the replacing patterns are also programmable for the user.
It will be clear that although the original colors are distinguishable in an image converted in such a manner, the quality of the image suffers serious impairment. In addition, the distribution of the color range over the different code patterns is pre-programmed and therefore not necessarily suitable for every color image. Consequently it may still happen that colors which are satisfactorily distinguishable from one another are printed with the same pattern.
A conversion of color values to gray values in a way which can be controlled by a user is described in U.S. Pat. No. 5,898,819, in which a solution is selected in which an image is divided up into document components, such as text, logos, images and so on. The user can choose a separate solution for each component. Gray scaling can also be selected, and this is equivalent in practice to averaging of the color values. In all these cases, pre-programmed conversion functions are involved, which are not adaptable to the image used. Here again, colors which are satisfactorily distinguishable may be imaged with the same gray value.